由于快速排序具有不稳定性,最好的时间复杂度为o(nlogn),而最坏可达到o(n^2),为了降低最坏情况出现的概率,可以用舍伍德算法对其进行改进~
#include<iostream> #include<stdlib.h> #include<time.h> #include<cstdio> #include<algorithm> #define N 1000 using namespace std; int a[N]; int Partion(int l,int r) { int key=a[l]; int i=l,j=r+1; while(1) { while(a[++i]<key&&i<=r); while(a[--j]>key&&j>=l); if(i>=j) break; if(a[i]!=a[j]) swap(a[i],a[j]); } if((j!=l)&&(a[l]!=a[j])) swap(a[l],a[j]); return j; } int Randpartion(int l,int r) { srand(time(NULL)); int k=rand()%(r-l+1)+l; swap(a[k],a[l]); k=Partion(l,r); return k; } void Quick_sort(int l,int r) { if(l<r) { int k=Randpartion(l,r); Quick_sort(l,k-1); Quick_sort(k+1,r); } } int main() { int T; cin>>T; while(T--) { int n; cin>>n; for(int i=1;i<=n;++i) cin>>a[i]; Quick_sort(1,n); for(int i=1;i<=n;++i) cout<<a[i]<<" "; cout<<endl; }return 0; }
运用快速排序求第k大数和第k小数:
#include<iostream> #include<stdlib.h> #include<time.h> #include<cstdio> #include<algorithm> #define N 1000 using namespace std; int a[N],b[N]; int Partion(int a[],int l,int r) { int key=a[l]; int i=l,j=r+1; while(1) { while(a[++i]<key&&i<=r); while(a[--j]>key&&j>=l); if(i>=j) break; if(a[i]!=a[j]) swap(a[i],a[j]); } if((j!=l)&&(a[l]!=a[j])) swap(a[l],a[j]); return j; } int Randpartion(int a[],int l,int r) { srand(time(NULL)); int k=rand()%(r-l+1)+l; swap(a[k],a[l]); int ans=Partion(a,l,r); return ans; } int Quick_sort(int a[],int l,int r,int k) { int p=Randpartion(a,l,r); if(p==k) return a[k]; else if(k<p) return Quick_sort(a,l,p-1,k); else { int j=0; for(int i=p+1;i<=r;++i) b[++j]=a[i]; return Quick_sort(b,1,j,k-p); } } int main() { int T; cin>>T; while(T--) { int n,k; cin>>n>>k; if(k<1||k>n) {cout<<"-1"<<endl;continue;} for(int i=1;i<=n;++i) cin>>a[i]; cout<<Quick_sort(a,1,n,n+1-k)<<endl; }return 0; }
法二:
#include<iostream> #include<stdlib.h> #include<time.h> #include<cstdio> #include<algorithm> #define N 1000 using namespace std; int a[N],b[N],tot; int Partion(int l,int r) { int key=a[l]; int i=l,j=r+1; while(1) { while(a[++i]<key&&i<=r); while(a[--j]>key&&j>=l); if(i>=j) break; if(a[i]!=a[j]) swap(a[i],a[j]); } if((j!=l)&&(a[l]!=a[j])) swap(a[l],a[j]); tot=0; for(i=l;i<=r;++i) { tot++; if(i==j) break; } return j; } int Randpartion(int l,int r) { srand(time(NULL)); int k=rand()%(r-l+1)+l; swap(a[k],a[l]); int ans=Partion(l,r); return ans; } int Quick_sort(int l,int r,int k) { int p=Randpartion(l,r); if(tot==k) return a[p]; else if(k<tot) return Quick_sort(l,p-1,k); else return Quick_sort(p+1,r,k-tot); } int main() { int T; cin>>T; while(T--) { int n,k; cin>>n>>k; if(k<1||k>n) {cout<<"-1"<<endl;continue;} for(int i=1;i<=n;++i) cin>>a[i]; cout<<Quick_sort(1,n,n+1-k)<<endl; }return 0; }
法三:
#include<iostream> #include<stdlib.h> #include<time.h> #include<cstdio> #include<algorithm> #define N 1000 using namespace std; int a[N],b[N]; int Quick_sort(int l,int r,int k) { srand(time(NULL)); int p=rand()%(r-l+1)+l; swap(a[p],a[l]); int i=l; int tot=1; for(int j=i+1;j<=r;++j)//把大于基准点的值都放到它的左边 if(a[l]<a[j]) { tot++; swap(a[++i],a[j]); } swap(a[l],a[i]); if(tot==k) return a[i]; else if(tot>k) return Quick_sort(l,i-1,k); else return Quick_sort(i+1,r,k-tot); } int main() { int T; cin>>T; while(T--) { int n,k; cin>>n>>k; if(k<1||k>n) {cout<<"-1"<<endl;continue;} for(int i=1;i<=n;++i) cin>>a[i]; cout<<Quick_sort(1,n,k)<<endl; }return 0; }